Fluidized bed contactors are used in a wide variety of industries to promote chemical reaction and heat transfer between particulate solids and gases or liquids. Examples of specific applications are catalytic petroleum cracking, minerals beneficiation, flue gas cleanup, and metals transformation.
An example of the later is the production of UF.sub.4 from UO.sub.3, where particulate UO.sub.3 is reacted in a first fluidized bed reactor with H2 to form UO.sub.2, and then particulate UO.sub.2 is reacted in a second fluidized bed reactor with HF to form UF.sub.4.
Fluidization quality is an important concern in the operation of the above-described fluidized beds. The most desirable situation is to have relatively smooth bubbling, which promotes good gas-solids mass transfer while at the same time providing enough solids mixing to prevent large thermal gradients and agglomeration or sintering. When the bubbling is too quiescent (e.g., when the bed is at minimum fluidization), solids mixing may be insufficient to prevent agglomeration and mass-transfer may become rate limiting. Slugging, at the other extreme, produces rather poor gas-solids contacting and allows a large fraction of the inlet gas to pass through the bed unreacted. This condition is known as "bypassing". Moreover, slugging results in high solids entrainment rates.
Slugging has an adverse effect on process efficiency, in that it results in an excessive use of reactant gases and excessive carryover of solids to the fluidized bed reactor filters. In turn, these cause increased production of waste scrubber liquor and increased filter maintenance frequency. The later is of particular concern because it exposes personnel to radiation hazards. Also, slugging can lead to excessive use of HF reactant gas.
The most efficient contacting condition is that which produces sufficient agitation to prevent solids clumping while minimizing the degree of gas bypassing. Because it results in reduced bypassing, smooth bubbling is more desirable than slugging. The only other constraint is that the bubbling be of sufficient degree to provide adequate mixing.
For dense solids such as UO.sub.2 and UO.sub.3 smooth bubbling occurs over a relatively narrow range of gas flow, typically from just above the minimum fluidizing velocity to between 10 and 20% above it. Beyond this flow range the transition to slugging occurs rather abruptly. Predicting the precise gas flows where these dynamic transitions occur is difficult because of the variation in minimum fluidizing velocity with operating conditions (e.g., changes in bed temperature and particle size). Fluidization quality in the beds, based on the intensity of flow oscillations and the degree of gas-solids mixing, is of concern because of potentially adverse environmental, safety and health consequences.
Some variation in conditions is inevitable from batch to batch and during the treatment of each individual batch as the reactions release heat and particles shrink due to mechanical attrition. Thus, even if the gas flow is initially set at the correct value for a given batch, the bed is likely to drift away from the optimum fluidization state over the course of a run.
In the past, pressure measurements have been used to monitor conditions in fluidized beds. U.S. Pat. No. 4,336,227 to Koyama et al. discloses a fluidized bed reactor in which the pressure drop is measured. The ratio of two pressure drops is used to represent the ratio of the fluid velocity and minimum fluidizing velocity. The supply of fluidizing gas and particles are controlled based on pressure drop measurements.
U.S. Pat. No. 3,128,129 to Stine et al. discloses a system for controlling the circulation rate in a fluidized bed. Pressure differential measurement is used to control circulation and thus insure that the flow is in a desired direction.
U.S. Pat. No. 3,467,502 to Davis discloses a control system for carbon black reactors, in which pressure is measured upstream and downstream of an effluent forming area. A shut-off valve is operated in response to the measured pressure, thus providing a control loop.
U.S. Pat. No. 4,226,798 to Cowfer et al. discloses a control system for a fluidized bed reactor, in which a change in pressure drop is measured.
U.S. Pat. No. 3,164,440 discloses a fluidized bed reactor where pressure across the bed is monitored during the fluidization of UF.sub.4 particles.
U.S. Pat. No. 5,047,209 to Lenczyk discloses a control system which relies on measured temperature fluctuations. The standard deviation of temperature at predetermined intervals of time is computed as part of the control process.
None of the above-noted U.S. patents would appear to provide an improved fluidization quality for fluidized beds prone to slugging based on determining fluidization quality and making corrective measures based on the determined quality.